This invention relates to a method of forming threshold switching devices which exhibit negative differential resistance and to the devices formed thereby. The method comprises depositing a silicon dioxide film derived from hydrogen silsesquioxane resin between at least two electrodes and applying a voltage above a threshold voltage across the electrodes. Such devices are useful as variable resistors.
Numerous devices which exhibit threshold switching are known in the art. For example, Ovshinsky in U.S. Pat. No. 3,271,591 describes such devices in which semiconductor materials, such as crystalline or amorphous tellurides, selenides, sulfides or oxides of substantially any metal, are deposited between electrodes. The semiconductors and methods specifically set forth in this reference, however, are not the same as those claimed herein. As such, the j-V curves in this reference differ from those of the present application.
Threshold switching with negative differential resistance is also known in various metal oxide thin films. For instance, Bullot et al., Phys. Stat. Sol. (a) 71, K1 (1982), describe threshold switching in vanadium oxide layers deposited from gels; Ansari et al., J. Phys. D:Appl. Phys. 20 (1987) 1063-1066 describe threshold switching in titanium oxide films formed by thermally oxidizing a titanium metal layer; Ramesham et al., NASA Tech Briefs, December 1989, p. 28, describe the switching in manganese oxide films and Morgan et al., Thin Solid Films, 15 (1973) 123-131, describe switching and negative differential resistance in aluminum oxide films. The materials and characteristics described in these references, however, differ from those described herein.
The switching and negative differential resistance characteristics of silicon oxide films have likewise been described. For instance, Simmons, Handbook of Thin Film Technology, Chapter 14 (1970), describes electronic conduction through thin insulating films, including silicon oxide, as well as their negative resistance and memory characteristics Al-Ismail et al., J. Mat. Sci. 20 (1985) 2186-2192, describe switching and negative resistance in a copper-silicon oxide-copper system; Morgan et al., Thin Solid Films, 20 (1974) S7-S9, describe threshold switching and memory in silicon oxide films; Boelle et al., Applied Surface Science 46 (1990) 200-205, describe the current voltage characteristics of silica films derived from sol-gel low temperature methods; and Klein, J. Appl. Phys., 40 (1969) 2728-2740, describe the electrical breakdown of silicon oxide films. As with the prior metal oxide references, however, these too do not describe the methods and characteristics described herein.
Resistors produced form ceramic oxides are also known in the art. For instance, Eijnthoven et al. and Nagano et al. describe resistors derived primarily from zinc oxide and titanium oxide, respectively. Such materials and the resultant properties, however, differ from those claimed herein.
Thin film silica coatings derived from hydrogen silsesquioxane resin are also known in the art. For instance, Haluska et al. in U.S. Pat. No. 4,756,977 describe forming such films by diluting hydrogen silsesquioxane resin in a solvent, applying the solution to a substrate, drying the solvent, and heating. Such coatings are taught therein to provide protection and electrical insulation.
The present inventors have now found that switching devices with desirable features can be formed by depositing a thin, hydrogen silsesquioxane derived silicon dioxide film between at least 2 electrodes and applying a voltage above a threshold voltage across the electrodes. These devices are useful as variable resistors.